1. Field of the Invention
The present invention relates to chassis measuring, and in particular a method and a device for checking the referencing of measuring heads of a contactless chassis measuring system.
2. Description of Related Art
In optical chassis measuring, for example, the measuring of toe and camber on motor vehicles, measuring heads are used, which each detect one wheel or possibly the wheels of one vehicle side. The position of wheel axes, rotational axes, wheel centers, or rotational centers are calculated from the measured values and used for determining the toe and camber.
A fundamental requirement in a system for, chassis and axle measuring is that the measured values of all measuring heads are provided in a shared coordinate system (“reference system”) or are transformed into such a shared coordinate system. The characteristic variables of the geometrical position of the measuring heads among one another, such as their position and orientation, which are required for a representation of the measured values in a shared coordinate system, are referred to hereafter as referencing of the measuring heads.
Various methods exist for generating a shared coordinate system in connection with the axle measuring:
In mechanical approaches (see, for example, published German patent document DE-3904557), mechanical adapters which determine the toe and the camber of an individual wheel are fastened directly onto the wheel. To produce a shared reference system, for example, additional optical sensors are integrated into the adapters, which are capable of determining the orientation with respect to adapters which are fastened on adjacent wheels.
Contactless approaches are characterized in that no adapters having measuring function are fastened onto the wheel. The measuring of the desired angular variables is typically executed by a measuring head, which has a video camera. The contactless approaches exist in various variants:
In a first variant, the referencing is only determined upon the construction of the system and is viewed as constant for all following measurements (see, for example, U.S. Pat. Nos. 4,745,469, 5,870,315).
In a second variant, each wheel is observed via a separate measuring head having a single or stereo video sensor. The values of the axle measuring are initially only provided in the local measuring system of the individual measuring head. For transforming the measuring values into a shared reference system, points are measured on a control point body before and during the measurement, each camera always only having to see one part of the control point body. A transformation into the shared control point system may be determined for each measuring head from the measurement of the control points in the local reference system of the individual measuring system and the known coordinates of the control points in the shared control point system. This referencing is typically performed before each measurement (see, for example, published German patent documents DE 3618480, DE 19757763, and DE 10050653, as well as US Patent Application Publication 2006 0279728).
In a third variant, an individual measuring head is expanded by an additional camera or a calibration element, which is in a calibrated relationship to the local coordinate system of the measuring head, to produce a shared reference system. The orientation between measuring head and additional camera is typically determined only once and is viewed as constant for all following measurements (see, for example, published European patent document EP 1184640, U.S. Pat. No. 6,968,282, and published European patent document EP 1309832).
The methods known up to now for referencing are imprecise and/or complex. It is therefore an object of the present invention to provide a method and a device for checking precisely and in a rapidly executable manner the referencing of measuring heads of a chassis measuring system, without increasing the hardware outlay.